I have a very simple macro on an Excel2007 workbook, that reads:
Sub MyMacro
msgbox "Hello world"
End Sub

Having added a customUI to this workbook (.xlsm), with a button that reads:
<button id="customButton" label="Run my macro" onAction="MyMacro()" /

.... when I press the button from the ribbon, the macro runs twice, and I get
a dialogbox that shows a heading of: "Microsoft Visual Basic", and a message
that simply reads "400" (a "critical" icon is also displayed)

I've opened the xlsm file (by renaming it as .zip), and have been through
all the xml files, and there is only one reference to the macro throughout
all the files. Also, the workbook itself as only the one macro, no other code.

I have also restarted my PC incase there was a memory leak or other
corruption, but still the same problem

Has anyone had a similar problem? Is there a fix?

I woudl gladly go back to Office2003, but my office have made the shift to
2007, so I must get this code working

Thanking you in advance
Paul